The present invention is concerned with a device for preliminary crushing of a possibly clogged, finely divided material to be ground and for feeding of the material into a jet mill containing a compressed flow of circulating grinding gas and material, so as to produce ultra-fineground admixtures to be used in industry.
In the prior art, exclusively ejector-type or injector-type feeder devices were used as feeder devices of jet mills of this type, the feeders being quite unsatisfactory in many respects, especially in view energy economy and of operation. In these prior-art devices the shifting of the material into the pressurized mill takes place by means of the grinding gas. In order to produce ultra-fineground particles, the most appropriate grinding gas is water vapour, for example the water vapour obtained from the outblow of a pulp mill boiler in accordance with the Finnish Patent Application 803,256.
During feeding, the grinding gas jet of the ejector sucks considerable quantities of air through the feeding funnel into the jet mill along with the material, which air has an adverse effect on the grinding result and on the recovery of heat from the grinding gas after the grinding. The higher viscosity of the air present during the grinding makes the grinding heterogeneous, which in particular makes the classification of the ground material more difficult.
Such a prior-art feeding device additionally imposes high requirements on the consistency of the material to be ground. The material should be preferably dry, and it should definitely not be clogged, whereas clogging, however, frequently occurs especially in winter, when the material is frozen during intermediate storage. In such a case the material must be crushed carefully before passing into the feeding funnel of the jet mill, which frequently results in unnecessary interruptions in operation.
An ejector feeder consumes about 20 percent of the total energy consumption of the grinding process.